Apparatus for recording seismic waves



Jan. 25, 1944. IM. D. MccAR'rY Y 2,340,272

APPARATUS FOR RECORDING SEISMIC WAVES original Filed May 4, 1940 ifa D!@gr #Y T/#EREA/f mkh y I j iA' A A Z' #maur/0415 @@PWHWI 7 @Bs Mm? ofexplosives.

Patented Jan. 25, 1944 Malcolm D. McCarty, Dallas, Tex., assignor, bymesne assignments, to Socony-Vacuum Oil Company, Incorporated, New York,N. Y., a corporation oi' New York Original application May 4,1 1940,Serial No.

N0. 333,301. Divided 3 (Ilaims.`

'Ihis application is a division of my copending Y and April 30, 1941,Serial No. 391,065

this application a considerableperiod of time subsequent to theapplication Serial No. 833,301, illed May 4, 1940. A

This invention relates generally to apparatus for recording seismicwaves and more particularly to apparatus for recording all of thedesired data that has been produced by the detonation of a single chargeof explosives, on a single trace of a seismogram.

In conducting geophysical surveys by seismic methods it has been thepractice heretofore to record three diierent groups of data that areobtainable from the seismic waves generatedin the earths surface by thedetonation of a charge One of these groups comprises data composed ofsignals that have been generated in sympathy with the seismic waveswhich travel vertically upward from the` explosive charge to the surfaceof the earth Where they are detected. From this data the velocity ofwave travel in the unconsolidated, uppermost strata of the earth isdeterminable. The second group of data comprises signals that vtraveldirectly through the unconsolidated 'stratum of the earths surface tothe geophones of a conventional spread. From this data the thickness ofthe unconsolidated weathered layer can be ccmputed. The third group ofdata comprises signals which have been generated in sympathy withseismic waves which travel downwardly from the point of detonation ofthe explosive charge to the interfaces of subsurface strata from whichthey are reected and detected at the earths surface by the geophones inthe conventional spread. U

Inasmuch as all of the above mentioned data are in the form of timeintervals counted from the instant of detonat'ion of the explosivecharge which creates the seismic waves, there of necessity, must be onevery record an indication of the exact instant of detonation of theexplosive charge. In the language of the art, this indication is knownas the time break impulse.

The above described data is recorded in the form of a seismogram havinga plurality of traces which have been recorded by means oi a recordinggalvanometer on a moving photographic lm or a sensitized lpaper. By theapparatus of the prior art, in addition tothe time break impulse, therewould be recorded on one of the traces a record corresponding to waveswhich have traveled vertically upward from the explosive charge andwhich have been detected by a geophone positioned adjacent the mouth ofthe shot hole, known in the art as' the uphole geophone. Since theuphole geophone will be energized for detection of the first waves, thetrace on which its record was recorded could not be used for recordingany of the other data necessary to be taken from that particular shot.Therefore, one of the vibrating elements of the recording galvanometerhad to besacriilced in order to make the record from the upholegeophone. Since only the first impulse detected by the uphole geophoneis used, the remainder of this trace is useless to the interpreter. Theother traces on the seismogram will have recorded on them signalsgenerated by the geophones in the conventional spread and these signalswill represent' waves which have traveled directly from the shot to thegeophones and waves which have traveled downwardly to the interfacesl ofthe substrata and have been reflected back to the surface.

Since the weight and size of seismographic recording equipment limitsthe number of vibrating elements that it is practical to use, it isdesirable to use all of the vibrating elements for recording the directtraveling waves and reected waves. Therefore, it is the primary objectof this invention to provide apparatus whereby the first impulses fromthe uphole geophone and all signals from one of the geophones of theconventional spread can be recorded on a single trace of a seismogram insuch a manner that one group of signals will not overlap or obscure theother groups.

Another object of this invention resides in the provision of meanswhereby the time break impulse, first impulse detected by the upholegeophone, waves traveling directly from the shot to a geophone in thespread, and reflections from subsurface interfaces detected by the samespread geophone can all be recorded on a single trace of a seismogramwithout any one overlapping or obscuring the others.

Still another object of this invention resides in the provision of meansfor rendering the uphole geophone inoperative after the first impulsesof vertically upward traveling waves have been detected, amplified andrecorded.

This invention further contemplates means associated with theconventional blasting machine for rendering the uphole geophoneinoperative after the vertically upward traveling waves have beendetected.

Another object of this invention resides in theprovision of meanswhereby the.uphole geophone and a geophone of the conventional spreadcan be connected to amplifying and recording means in such a manner thatthe uphole geophone can be rendered inoperative after the first impulsesfrom the vertically upward traveling waves have been detected andrecorded without molesting the other geophone.

Still another object of this invention resides in the provision of meanswhereby the uphole geophone can be shunted a predetermined period oftime after the detonation of the explosive charge, this predeterminedperiod of time being just long enough to allow the first of thevertically upward traveling waves to reach the uphole geophone and bedetected, amplified .and recorded.

Other objects and advantages of the invention will be apparent from thefollowing detailed description when considered with the drawing inwhich:

Figure 1 is a schematic diagram representing a seismograph operationsuch as that to which the present invention appertains;

Figure 2 is a circuit diagram Aof a modified blasting machine showingthe manner in which the uphole geophone is shunted 'by the operatic ofthe plunger of the blasting machine. I

Figure 3 is a modification oi.' the circuit diagram shown in Figure 2 inthat it discloses means for breaking the circuit of the uphole geophonea predetermined time after closing the detonating circuit; and

Figure 4 illustrates a trace.of a` seismogram that has been recorded inthe manner taught by the present invention on which is recorded the timebreak impulse, the rst impulses detected by the uphole geophone, thedirect traveling waves that have been detected by a geophone of theconventional spread after the uphole geophone has been renderedinoperative, and subsequent reflections. i

Referring to the drawing in detail particularly Figure 1, there is showndiagrammatically a complete set of seismograph equipment and a sectionthrough the earths surface showing the disposition of the explosive andthe paths traveled by the many waves that radiate therefrom. A shot orexplosive charge I is detonated through the conductors II by means ofthe lblasting machine I2. The detonation of the explosive charge I0generates seismic waves in the earths surface which travel outwardlyfrom the point of disturbance in the form of a spherical wave front.Some of these waves shown at a will travel vertically upward and impingeuponA a geophone Gh positioned adjacent the mouth of the shot hole I3.These waves impinging upon the geophone Gn will generate correspondingvoltage waves. These voltage waves will be transmitted through theconductors I4, the winding of lgeophone G1 when it is connected inseries with the geophone Gn, conductors I 5 to the amplier A1 where theywill be amplified and transmitted by means of the conductor I6 to therecording galvanometer R. After the first impulse, or impulses have beendetected by the geophone Gil the blasting machine I2 and its associatedelements forming the instant invention will render this geophoneinoperativeand as a result no further signals after a predetermined timehas elapsed will pass through the conductors I4 to be eventuallyvrecorded by the galvanometer R.

At the same time that waves a are being transmitted vertically upward,other waves will be transmitted along the paths b substantially directlyto the geophones G1, G2, and G3. These waves will cause these geophonesto generate corresponding electrical signals which will be communicatedthrough the conductors I5, I1,'and. I8 to the amplifiers A1, A2, and A3where they will be amplified and transmitted through the conductors I6,I9, and 20 to their respective vi-v brating elements in the recordinggalvanometer R.

Simultaneously with the generation of the waves which follow the pathsindicated by a and b, other waves will travel downwardly to strike theinterfaces of Substrata from which they will be reected along the pathsindicated at c and d. These reflected waves will also -be detected bygeophones G1, Gz, and G3, and corresponding voltage Waves will `begenerated. These signals in the manner described above in connectionwith those waves traveling vertically upward from their point ofcreation and those traveling directly to the geophones G1, G2, and G3will be amplified and recorded on the recording galvanometer R.

In order to measure the period of time required for these waves totravel from their point of creation to the geophones by the pathsenumerated above means are provided in the blasting machine circuit I2and the communication circuit, formed by the conductorY 2| and the timebreak andY communication receiving unit 22, whereby a signal indicativeof the instant of detonation can be transmitted through conductors 23 toone trace of the recording galvanometer R. In addition to a signalindicative of the instant of detonation, other means must be providedproducing indications of the passage of time on the seismogram.

These indications are usually in the form of parallel transverse linesspaced 1/100 of a second apart.

As pointed out above it is desirous to record a time break signal, anuphole geophone impulse, first breaks due to direct traveling waves andsignals corresponding to waves which have traveled downward to theinterfaces of the substrata and have been reflected upwardly to thesurface of the earth where they are detected Aon a single trace of aseismogram. A trace of a seismogram illustrating such a recording isshown in Figure 4. In order to record such a trace, means arel provided,as disclosed in Figures 2 and 3, for either breaking the circuit throughthe geophone G11 or shunting it at a predetermined interval of timeafter the detonation of the explosive charge. The time break is shown onthe trace at 2Q, the uphole` geophone impulse at 25, the first break ofdirect traveling wave energy at 26 and reflections at 21. Utilizing theinstant invention, it can be seen that all of the elements of a multipleelement galvanometer can be used to record the reflections fromindependent geophones while at the same time producing an accuraterecord of the time break, uphole geophone impulse, and first breakscorresponding to the f'lrst energy detected of direct traveling waves.

In, order to accomplish the above means are `provided in conjunctionwith a blasting madrive the generator 29.- the 'explosive charge will bedetonated when the switchA 33 is closed as the bottom end of the plungerpasses the cam 36. The operation of the blasting machine described thusfar is that of the conventional plunger type blasting machine. I

The geophones G1. G2, and Ge illustrated in Figures 1, 2 and 3 are thoseused in the conventional seismic reilection spread. In order to detectseismic waves such as those illustrated at a in Figure 1, those waveswhich travel vertically upward from the shot, a geophone Gn is locatedadjacent the mouth of the shot hole. This geophone, as shown in Figure2, is connected in series with one of the geophones G1 of theconventional spread. In Figure 3 it is shown connected in parallel witha geophone G1 ofthe conventional spread.' With the arrangements shown inFigure 2 the uphole geophone Gn is operative to detect the waves which`travel vertically upward from the shot at the time detonation ofthe shotoccurs. In order to render this geophone inoperative a predeterminedperiod of time after detonation ofvthe explosive charge occurs a secondswitch 31 is provided in the blasting machine. Switch 31 is of the sametype as switch 33 and is cam operated by the plunger in the same manner.This second switch is spaced suiciently far below switch 33 that aperiod of time long enough for the vertically upward traveling wavesl tobe detected bythe geophone Gh elapses before thebottom end oi' theblasting machine plunger 28 strikes the cam 38 of switch 31 to close theswitch. Closing of switch 31 by the plunger as it reaches a pointsubstantially near the end of its stroke places a shunt formed by theyconductors 39 and 40 and the switch 31 directly across the geophone Gnto render it inoperative. With geophone Gn shunted as described above,signals generated by -it subsequent to the closing of switch 31 will notbe effective to disturb the seismogram trace on which the time break anduphole signals are recorded. The trace from this instant of time on isavailable for recording data corresponding to the rst break occasionedby waves which travel directly from the shot to the geophone G1 andsubsequent signals generated by the geophone G1 which correspond towaves which have been reflected from the interfaces of the substrata.

Another embodiment of this invention is illustrated in Figure 3. In thiscase a geophone Gn is initially connected in parallel with the geophoneG1 ofthe conventional spread. The detonating circuit through theblasting machine is completed in the same manner as described .inconnection with Figure 2.l In this instance, however. the plunger 28after closing the switch 33 in the detonating circuit. on furtherdownward movement, opens a switch 30 to break the circuit through theuphole geophone Gn. Switch 40 is normally closed, but as the plunger 28reaches the point substantially near the bottom of its stroke, itscontact with cam 3| causes the arm 42 of the switch 40 to move outwardlyaway from contact 43 to. open the circuit through Gn and render itinoperative. The distance between the switch 33 and switch 40 is soselected that ample time between the closing of switch 33 and theopening of switch 40 for the waves traveling vertically upward to bedetected by Gn will have elapsed. After the circuit through geophone Gnhas been broken thereby rendering it inoperative to transmit signals tothe recorder only signals from the geophones G1 of the conventionalspread will be recorded on this trace oi the seismogram.

From the above it becomes readily apparent to those skilled in the artthat all the data that is necessary from a single spread when shot inone direction can be recorded on a single seismogram, therebyeliminating the necessity for recording additional shots or for usinga.single trace for recording no more than the uphole geophone impulseand/or the time break and uphole geophone impulse. y

Referring to Figure 4 a period of elapsed time T1 represents the timerequired for waves to travel vertically upward from the shot'to thesurface of the earth where they are detected by the geophone Gn. Theperiod of elapsed timeTa represents the interval between the closing ofswitch 33 and the closing of switch 31, or the opening of switch 40 asthe case may be, to render the uphole geophone Gh inoperative. Thisperiod of time is controllable by the vertical spacing of switches 33and 31 or switches 33 and 40. The period of time T3 represents theelapsed time between the instant of detonation and the detection ofdirect traveling waves such as those illustrated at b in Figure 1. Afterthe direct traveling waves have been detected by the geophone Gi of theconventional spread by employing suitable expanders or automatic volumecontrol means signals 21corresponding to reflected waves can be lrecorded subsequently on the same trace or traces with usable amplitude.l

Itis obvious to those skilled .in the art that by means of the apparatusof the instant application a record of the uphole geophone impulse canbe recorded on any trace of the seismogram Without interfering with theadditional data that it is de sired to have recorded on the particulartrace.

1. An electric seismographcomprising in combination, means for creatingseismic waves in the earth at a point substantially below the surface, aconventional geophone spread, amplifiers for each of the geophones, anda multiple element recorder for recording signals generated by thegeophones in the spread, an additional geophone located on the earthssurface directly above the point where the waves are created in theearth, means for recording signals generated by the last mentionedgeophone on the same recorder element as signals from a geophone in thespread, means operable by an -element of the blasting machine forrendering this geophone inoperative a predetermined time after thecreation of the data that is required from a single geophonel spreadwhen shot in one direction can be `recorded on a single seismogram.

2. An electric seismograph comprising in combination, means for creatingseismic waves in the means operable by an element of the blastingmachine for electrically shunting this geophone a signals generated by aspread geophone will not be obscured by signals generated by thegeophone that is located directly above the point o creation of theseismic waves when recorded on the same trace.

3. An electric seismograph comprising :ln combination, means forcreating seismic waves in the earth at a point substantially below thesurface, a blasting machine for detonating said means, a conventionalgeophone spread, amplifiers for each of the geophones and a multipleelement recorder for recording signals generated. by geophones in thespread, an additional geophone located on the earth's surface directlyabove the point where the waves are created in the earth, means forcommunicating signals generated by the last men tioned geophone to themultiple element recorder, means operable by an element of the blastingmachine for breaking a circuit through this geophone to render itinoperative a predetermined time after the creation of the seismic wavesbut before the detection of the seismic waves by the geophones inthe-spread whereby signals generated by a spread geophone will not beobscured by signals generated by the geophone that is located directlyabove the point of creation of the seismic waves when recorded on thesame trace.

MALCOLM D. MCCARTY.

